The present invention relates methods and apparatus for recovering volatile organic and inorganic compounds, including hazardous air pollutants, from vessels such as barges that need (1) vapor de-pressuring for changing products or for human entry for servicing and/or inspection, (2) liquid filling, or (3) liquid unloading. In more detail, the present invention relates to methods and apparatus for compressing, cooling, condensing, separating and storing volatile, and usually explosive, vapors with a product vapor pressure from below atmospheric pressure up to about 250 PSIG from the ullage of a vessel such as a barge during vapor de-pressuring, loading, or unloading of the vessel. The pressure of 250 PSIG was chosen for economic reasons as it includes valuable products with vapor pressures up to and including propane, propylene, and ammonia. In addition, the present invention strips the inert gas such as nitrogen from the product vapor to a high degree of purity and, by means of an automatic valve control system, recycles the inert gas that is recovered back to the vessel being serviced. This process greatly reduces the amount of expensive inert gas needed and can produce purity covered products.
Equipment is described in the patent literature, and a number of systems are currently in use, for the recovery of volatile hydrocarbons and chemical vapors released from the filling of tanks or barges with volatile liquids. For instance, U.S. Pat. No. 5,176,002 describes such a system that also recovers some of the inert gas by means of a column.
Previous attempts to recover hydrocarbon vapors displaced during the filling of storage tanks have utilized large condensation or absorption columns in combination with compressors and/or other apparatus to recover the hydrocarbon vapor as a liquid. Examples of such processes are disclosed in U.S. Pat. Nos. 3,967,938 and 4,10,091. A system for recovering and recycling pad gases discharged from marine vessels during loading is described in U.S. Pat. No. 5,524,456.
Another type of system for controlling hydrocarbon and other emissions during filling of storage tanks utilizes activated carbon or other solid adsorbent beds that selectively adsorb hydrocarbon vapors from the displaced vapor stream. The adsorbent bed is regenerated such as by a reduction in pressure to remove the hydrocarbon from the activated carbon. The hydrocarbon stream is then combusted or further processed in an absorption tower to recover a portion of the hydrocarbon as a liquid. An example of such a system is disclosed in U.S. Pat. No. 4,066,423.
A storage terminal vapor emission control system including an activated carbon absorption system and a plurality of vapor collection ducts connected in a way to effectively capture and control the vapors emitted is described in U.S. Pat. No. 4,995,890. Another system employing a vapor recovery system for loading racks and storage tanks that employs blanketing gas under controlled pressure conditions for admixture with vapor of the volatile liquid is represented. This mixture is processed for recovery of the vapor and gas constituents under the supervision of and control of precision oxygen monitoring. This system is described in U.S. Pat. No. 4,098,303.
However, on information and belief, no systems are known that perform all four of the following operations: 1) vapor de-pressuring, 2) liquid filling, 3) liquid unloading, and 4) inert gas recovery. It is therefore a primary object of the present invention to provide a method and apparatus for performing all four of these operations.
There are a number of factors that must be considered when a barge or other vessel is de-pressured for changing products or for such purposes as allowing human entry for servicing and/or inspection. In addition to the flammability of the vapors released from vessels that need to be entered, the creation of a breathable mixture of air must also be considered. It is recommended practice to have an expert monitor the lower explosive limit (LEL) of the vapors being purged and removed from barges and other vessels that need to be serviced before the hatches are opened. It is common practice to introduce an inert gas, most commonly nitrogen, into the vessel to be purged, such as a barge. The nitrogen can be introduced as a continuous purging flow of nitrogen where the barge discharge valve is open. Nitrogen can also be introduced as a batch flow where the barge pressure is increased, depending on the barge pressure rating, with the barge discharge valve closed. The barge pressure is then quickly released to help purge faster in a series of batch processes. A technician then makes precise lower explosive limit readings to determine when the barge has been sufficiently purged to enable the barge hatches to be opened to the air for servicing the barge without danger of explosion.
The common procedure for disposing of barge vapors prior to servicing the barge is to purge with nitrogen and pipe the vapors directly to a flare stack or to direct the vapor mixture to a low pressure vapor recovery system that mixes all recovered products. Vapors that are burned at the flare stack create no recovery value but, if not flared, these products are valuable if recovered as a pure liquid or as a mixture of liquids with a reasonably high BTU value for fuel. Many barge repair or inspection facilities receive barges with anywhere from 5 to 50 or more different hydrocarbon vapors or chemical vapors that need to be removed from the barges. These vapors will be either flammable hydrocarbons or harsh or hazardous chemical vapors. The vapor pressure of these barge products may vary from far below atmospheric pressure to over 250 PSIG vapor pressure. Many vapor recovery systems on the market today, such as rich oil-lean oil plants that are only designed to recover such products as benzene, alcohols and gasoline, do not operate above 5-10 PSIG vapor pressure. Products above that vapor pressure that are processed in such a system must be flared. High pressure, rich oil-lean oil plants are available, but are complex and expensive. Nitrogen is generally chosen as the inert gas used to purge vessels such as barges. Liquid nitrogen is relatively expensive but readily obtainable in sufficient purity (99.9% pure) to enable purging of the vessels to the 10% of lower explosive limit (LEL) purity required for human entry for welding operations. The LEL of propane, for example, is 2.0% propane in air. Nitrogen is difficult to recover in relative purity because of the cryogenic temperatures needed to separate it from the vapors being processed. In short, the design of such systems is almost always constrained by the specifics of each installation.
Many facilities must handle, or could benefit from handling, all four conditions listed earlier with a single, multi-purpose vapor recovery system. There is, therefore, a need for methods and apparatus that are adaptable for recovering a multitude of vapors from vessels to enable them to be de-pressured, filled or unloaded. There is a need for methods and apparatus that are safely used for moving vapors that are volatile, flammable, or hazardous from vessels that need to be processed. There is a need to recover the inert gas whenever economically possible. There is also a need to recover valuable vapors that are now being flared with absolutely no recovery. There is also a need to reduce facility-wide vented vapors that are commonly flared creating atmospheric byproducts, NOX, and noise and lighting nuisances.
These needs, and others known to those skilled in the art, are met by providing an apparatus for recovering vapors of a wide variety of vapor pressures from a vessel being de-pressured, filled or unloaded comprising a two-stage vapor compressor system with inlet separator, interstage cooler/condenser, after cooler, heat exchangers, oil separators, pumps, piping, storage tanks, dehydration system, lubrication system, automatic valve control system, control panel, and explosion proof electrical control system. The compressors, heat exchangers, and other components are designed to compress the specific vapor being processed to a specific storage tank pressure, to pump the condensed liquids to that specific storage tank, and to separate and recover a large part of the nitrogen and recycle it back to the vessel as a nearly pure nitrogen vapor.
In another aspect, the present invention provides a method of recovering vapor and the inert gas from the ullage of a vessel during operations such as de-pressuring or the loading and/or unloading of the vessel comprising the steps of:
(a) removing vapor from the ullage of the vessel;
(b) decreasing the temperature of the vapor to a temperature at which the liquids in the vapor condense by passing the vapor through a heat exchanger cooled by an inert expandable gas;
(c) separating the condensed liquids from the cold stream of vapor; and
(d) utilizing the stream of cold vapor from which the condensed liquids have been separated in step (c) to decrease the temperature of the vapor between steps (a) and (b).
In this manner, the vapors are recovered in sufficient purity to have value as, for instance, fuel and/or for subsequent re-use.